327 research outputs found
Superstrings with multiplicities
A superstring of a set of words P = s1, · · · , sp is a string that contains each word of P as substring. Given P, the well known Shortest Linear Superstring problem (SLS), asks for a shortest superstring of P. In a variant of SLS, called Multi-SLS, each word si comes with an integer m(i), its multiplicity, that sets a constraint on its number of occurrences, and the goal is to find a shortest superstring that contains at least m(i) occurrences of si. Multi-SLS generalizes SLS and is obviously as hard to solve, but it has been studied only in special cases (with words of length 2 or with a fixed number of words). The approximability of Multi-SLS in the general case remains open. Here, we study the approximability of Multi-SLS and that of the companion problem Multi-SCCS, which asks for a shortest cyclic cover instead of shortest superstring. First, we investigate the approximation of a greedy algorithm for maximizing the compression offered by a superstring or by a cyclic cover: the approximation ratio is 1/2 for Multi-SLS and 1 for Multi-SCCS. Then, we exhibit a linear time approximation algorithm, Concat-Greedy, and show it achieves a ratio of 4 regarding the superstring length. This demonstrates that for both measures Multi-SLS belongs to the class of APX problems. © 2018 Yoshifumi Sakai; licensed under Creative Commons License CC-BY.Peer reviewe
Linking BWT and XBW via Aho-Corasick automaton : Applications to run-length encoding
Peer reviewe
Pore evolution in interstellar ice analogues: simulating the effects of temperature increase
Context. The level of porosity of interstellar ices - largely comprised of
amorphous solid water (ASW) - contains clues on the trapping capacity of other
volatile species and determines the surface accessibility that is needed for
solid state reactions to take place. Aims. Our goal is to simulate the growth
of amorphous water ice at low temperature (10 K) and to characterize the
evolution of the porosity (and the specific surface area) as a function of
temperature (from 10 to 120 K). Methods. Kinetic Monte Carlo simulations are
used to mimic the formation and the thermal evolution of pores in amorphous
water ice. We follow the accretion of gas-phase water molecules as well as
their migration on surfaces with different grid sizes, both at the top growing
layer and within the bulk. Results. We show that the porosity characteristics
change substantially in water ice as the temperature increases. The total
surface of the pores decreases strongly while the total volume decreases only
slightly for higher temperatures. This will decrease the overall reaction
efficiency, but in parallel, small pores connect and merge, allowing trapped
molecules to meet and react within the pores network, providing a pathway to
increase the reaction efficiency. We introduce pore coalescence as a new solid
state process that may boost the solid state formation of new molecules in
space and has not been considered so far.Comment: 9 pages, 8 figures Accepted for publication in A&
The first frost in the Pipe Nebula
Spectroscopic studies of ices in nearby star-forming regions indicate that
ice mantles form on dust grains in two distinct steps, starting with polar ice
formation (H2O rich) and switching to apolar ice (CO rich). We test how well
the picture applies to more diffuse and quiescent clouds where the formation of
the first layers of ice mantles can be witnessed. Medium-resolution
near-infrared spectra are obtained toward background field stars behind the
Pipe Nebula. The water ice absorption is positively detected at 3.0 micron in
seven lines of sight out of 21 sources for which observed spectra are
successfully reduced. The peak optical depth of the water ice is significantly
lower than those in Taurus with the same visual extinction. The source with the
highest water-ice optical depth shows CO ice absorption at 4.7 micron as well.
The fractional abundance of CO ice with respect to water ice is 16+7-6 %, and
about half as much as the values typically seen in low-mass star-forming
regions. A small fractional abundance of CO ice is consistent with some of the
existing simulations. Observations of CO2 ice in the early diffuse phase of a
cloud play a decisive role in understanding the switching mechanism between
polar and apolar ice formation.Comment: 17 pages, 8 figures, accepted by A&
Design to reliability shielded vertical interconnection applied to microwave Chip Scale Packaging
This paper presents the electrical design, measurement and reliability ests of a shielded vertical interconnection dedicated to microwave solder-mount packages. Electromagnetic simulations show very good results up to 20 GHz. Test samples have been designed and manufactured. Electrical results are in accordance with the simulations with insertion loss lower than 0.1 dB up to 20 GHz for the proposed interconnection. Reliability tests of present no degradation of the after 500 thermal cycles in the [-55°C, +125°C] temperature range
Space-time evolution of electron cascades in diamond
Here we describe model calculations to follow the spatio-temporal evolution
of secondary electron cascades in diamond. The band structure of the insulator
has been explicitly incorporated into the calculations as it affects
ionizations from the valence band. A Monte-Carlo model was constructed to
describe the path of electrons following the impact of a single electron of
energy E 250 eV. The results show the evolution of the secondary electron
cascades in terms of the number of electrons liberated, the spatial
distribution of these electrons, and the energy distribution among the
electrons as a function of time. The predicted ionization rates (5-13 electrons
in 100 fs) lie within the limits given by experiments and phenomenological
models. Calculation of the local electron density and the corresponding Debye
length shows that the latter is systematically larger than the radius of the
electron cloud. This means that the electron gas generated does not represent a
plasma in a single impact cascade triggered by an electron of E 250 eV energy.
This is important as it justifies the independent-electron approximation used
in the model. At 1 fs, the (average) spatial distribution of secondary
electrons is anisotropic with the electron cloud elongated in the direction of
the primary impact. The maximal radius of the cascade is about 50 A at this
time. As the system cools, energy is distributed more equally, and the spatial
distribution of the electron cloud becomes isotropic. At 90 fs maximal radius
is about 150 A. The Monte-Carlo model described here could be adopted for the
investigation of radiation damage in other insulators and has implications for
planned experiments with intense femtosecond X-ray sources.Comment: 26 pages, latex, 13 figure
Finding all maximal perfect haplotype blocks in linear time
Recent large-scale community sequencing efforts allow at an unprecedented level of detail the identification of genomic regions that show signatures of natural selection. Traditional methods for identifying such regions from individuals' haplotype data, however, require excessive computing times and therefore are not applicable to current datasets. In 2019, Cunha et al. (Advances in bioinformatics and computational biology: 11th Brazilian symposium on bioinformatics, BSB 2018, Niteroi, Brazil, October 30 - November 1, 2018, Proceedings, 2018. 10.1007/978-3-030-01722-4_3) suggested the maximal perfect haplotype block as a very simple combinatorial pattern, forming the basis of a new method to perform rapid genome-wide selection scans. The algorithm they presented for identifying these blocks, however, had a worst-case running time quadratic in the genome length. It was posed as an open problem whether an optimal, linear-time algorithm exists. In this paper we give two algorithms that achieve this time bound, one conceptually very simple one using suffix trees and a second one using the positional Burrows-Wheeler Transform, that is very efficient also in practice.Peer reviewe
Porosity measurements of interstellar ice mixtures using optical laser interference and extended effective medium approximations
Aims. This article aims to provide an alternative method of measuring the
porosity of multi-phase composite ices from their refractive indices and of
characterising how the abundance of a premixed contaminant (e.g., CO2) affects
the porosity of water-rich ice mixtures during omni-directional deposition.
Methods. We combine optical laser interference and extended effective medium
approximations (EMAs) to measure the porosity of three astrophysically relevant
ice mixtures: H2O:CO2=10:1, 4:1, and 2:1. Infrared spectroscopy is used as a
benchmarking test of this new laboratory-based method. Results. By
independently monitoring the O-H dangling modes of the different water-rich ice
mixtures, we confirm the porosities predicted by the extended EMAs. We also
demonstrate that CO2 premixed with water in the gas phase does not
significantly affect the ice morphology during omni-directional deposition, as
long as the physical conditions favourable to segregation are not reached. We
propose a mechanism in which CO2 molecules diffuse on the surface of the
growing ice sample prior to being incorporated into the bulk and then fill the
pores partly or completely, depending on the relative abundance and the growth
temperature.Comment: 9 pages, 6 figures, 1 table. Accepted for publication in A&
Near-arcsecond resolution observations of the hot corino of the solar type protostar IRAS 16293-2422
Complex organic molecules have previously been discovered in solar type
protostars, raising the questions of where and how they form in the envelope.
Possible formation mechanisms include grain mantle evaporation, interaction of
the outflow with its surroundings or the impact of UV/X-rays inside the
cavities. In this Letter we present the first interferometric observations of
two complex molecules, CH3CN and HCOOCH3, towards the solar type protostar
IRAS16293-2422. The images show that the emission originates from two compact
regions centered on the two components of the binary system. We discuss how
these results favor the grain mantle evaporation scenario and we investigate
the implications of these observations for the chemical composition and
physical and dynamical state of the two components.Comment: 5 pages (apjemulate), 2 figures; accepted by ApJ
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